1. Field of the Invention
The invention relates to an exhaust gas apparatus of an internal combustion engine, in particular an internal combustion engine with direct gasoline injection, having at least one selective catalytic converter. The invention further relates to a method for cleaning the exhaust gas from an internal combustion engine, in particular an internal combustion engine with direct gasoline injection, in particular for use in an exhaust gas apparatus.
2. Description of the Prior Art
The fuel consumption of Otto engines, especially Otto engines with direct gasoline injection, can be reduced markedly in the partial-load range with the aid of combustion with air excess and by means of charge stratification. Pronounced advantages in combustion in the lowest load range (to some extent above 30%) and in the relevant travel cycle can be achieved by a reduction in intake throttle losses, a thermodynamically more-favorable gas composition, and in certain ranges, reduction of the wall heat losses. However, the lean mixture composition prevents a desired high reduction of nitrogen oxides in the three-way catalytic converter, and for that reason, to achieve more-favorable exhaust gas values, special NOx reservoir systems and deNOx catalytic converters are used. The prerequisite is low-sulfur fuel, but it is not extensively available worldwide, which restricts its use in certain markets. Moreover, the advantage in efficiency attained by charge stratification is reduced by the necessary regeneration of the deNOx catalytic converter, and there are additional cost disadvantages in comparison to conventional exhaust gas posttreatment systems.
One effective method for removing nitrogen oxides (NOx) in oxygen-containing exhaust gases is the possibility of reducing these nitrogen oxides to nitrogen and water via selective catalytic reaction (SCR). As the reducing agent, ammonia (NH3) is used, which in mobile applications for safety reasons is carried in the vehicle in only slight quantities and released as needed in small quantities from an NH3 buffer store, such as a urea-water solution (or AdBlue™) or from solids, such as ammonium carbamate, ammonium carbonate, and the like, and is metered into the exhaust gas. In the diesel utility vehicle sector, such systems are already in use.
However, in Otto engines, especially those with direct gasoline injection, such systems cannot be used for lack of high-temperature resistance of the selective catalytic converters. Moreover, the selective catalytic process is impeded vulnerably by the hydrocarbons that are present in the exhaust gases from Otto engines. Precisely at high load, direct gasoline injection engines are not run lean but rather with what is called a stoichiometric air ratio (λ=1 concept), with the consequence of relatively high nitrogen oxide and hydrocarbon emissions. Cold starting also takes place in the λ=1 mode, to avoid misfiring at cold engine temperatures.